Isotopic ratio of radioactive iodine (¹²⁹I/¹³¹I) released from Fukushima Daiichi NPP accident

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In March 2011, there was an accident at the Fukushima Daiichi Nuclear Power Plant (NPP) and a discharge of radionuclides resulting from a powerful earthquake. Considering the impact on human health, the radiation dosimetry is the most important for ¹³¹I among radionuclides in the initial stage immediately following the release of radionuclides. Since ¹³¹I cannot be detected after several months owing to its short half-life (8 days), the reconstruction by ¹²⁹I (half-life: 1.57 × 10⁷ yrs) analysis is important. For this reconstruction, it is necessary to know the isotopic ratio of ¹²⁹I/¹³¹I of radioactive iodine released from the NPP. In this study, the ¹²⁹I concentration was measured in several surface soil samples collected around the Fukushima Daiichi NPP for which the ¹³¹I level had already been determined. The surface deposition amount of ¹²⁹I was between 15.6 and 6.06 × 10³ mBq/m² within the region 3.6 to 59.0 km distant from the NPP. ¹²⁹I and ¹³¹I data had good linear correlation and the average isotopic ratio was estimated to be ¹²⁹I/¹³¹I = 31.6 ± 8.9 as of March 15, 2011.